1. Field of the Invention
The present invention relates generally to fuel cell systems, and, more particularly, to water management in a fuel cell system.
2. Description of the Related Art
Electrochemical fuel cells convert reactants, namely fuel and oxidant fluid streams, to generate electric power and reaction products. Electrochemical fuel cells generally employ an electrolyte disposed between two electrodes, namely a cathode and an anode. An electrocatalyst, disposed at the interfaces between the electrolyte and the electrodes, typically induces the desired electrochemical reactions at the electrodes. The location of the electrocatalyst generally defines the electrochemically active area.
One type of electrochemical fuel cell is the proton exchange membrane (PEM) fuel cell. PEM fuel cells generally employ a membrane electrode assembly (MEA) comprising a solid polymer electrolyte or ion-exchange membrane disposed between two electrodes. Each electrode typically comprises a porous, electrically conductive substrate, such as carbon fiber paper or carbon cloth, which provides structural support to the membrane and serves as a fluid diffusion layer. The membrane is ion conductive, typically proton conductive, and acts both as a barrier for isolating the reactant streams from each other and as an electrical insulator between the two electrodes. A typical commercial PEM is a sulfonated perfluorocarbon membrane sold by E.I. Du Pont de Nemours and Company under the trade designation NAFION®. The electrocatalyst is typically a precious metal composition (e.g., platinum metal black or an alloy thereof) and may be provided on a suitable support (e.g., fine platinum particles supported on a carbon black support).
In a fuel cell, an MEA is typically interposed between two separator plates that are substantially impermeable to the reactant fluid streams. Such plates are referred to hereinafter as flow field plates. The flow field plates provide support for the MEA. In addition, the flow field plates have channels, trenches or the like formed therein which serve as paths to provide access for the reactant and the oxidant fluid streams to the respective porous electrodes. Also, the fluid paths provide for the removal of reaction byproducts and depleted gases formed during operation of the fuel cell.
In a fuel cell stack, a plurality of fuel cells are connected together, typically in series, to increase the overall output power of the fuel cell system. In such an arrangement, one side of a given separator flow field plate may be referred to as an anode flow field plate for one cell and the other side of the plate may be referred to as the cathode flow field plate for the adjacent cell.
A plurality of inlet ports, supply manifolds, exhaust manifolds and outlet ports are utilized to direct the reactant fluid to the reactant channels in the flow field plates. The supply and exhaust manifolds may be internal manifolds, which extend through aligned openings formed in the flow field plates and MEAs, or may comprise external or edge manifolds, attached to the edges of the flow field plates. A variety of configurations are possible.
A broad range of reactants can be used in PEM fuel cells. For example, the fuel stream may be substantially pure hydrogen gas, a gaseous hydrogen-containing reformate stream, or methanol in a direct methanol fuel cell. The oxidant may be, for example, substantially pure oxygen or a dilute oxygen stream such as air.
During normal operation of a PEM fuel cell, fuel is electrochemically oxidized on the anode side, typically resulting in the generation of protons, electrons, and possibly other species depending on the fuel employed. The protons are conducted from the reaction sites at which they are generated, through the membrane, to electrochemically react with the oxidant on the cathode side. The electrons travel through an external circuit providing useable power and then react with the protons and oxidant on the cathode side to generate water reaction product.
As noted above, the channels of the flow field plate serve two functions: to deliver reactants/oxidants to the active region of the membrane, and to remove byproducts and depleted gasses resulting from the electrochemical generation process. When hydrogen is used as the reactant and oxygen is used as the oxidant, one of the byproducts of the electrochemical generation process is water. Although some amount of water in the active region of the membrane is desirable, the accumulation of water can result in undesirable amounts of water in some regions of the fuel cell.
Water may accumulate in the flow field channels. As gas is injected into the flow field plate channels (reactants and/or oxidants), gas pressure and movement may “flush” the accumulated water through the above-described outlets.
If a relatively large amount of water collects in a localized region of a flow field plate channel, however, the channel may become blocked by the water. If the channel becomes blocked by accumulated water, gas flow stops. Consequently, as the reactants and/or oxidants in the gas residing in the blocked channel are depleted, electrical output of the fuel cell decreases. Maintaining fuel cell efficiency is very desirable.
In some fuel cell systems, other problems may arise from the accumulation of water in and around the membrane. Droplets of water may divert gas flow into less-than-optimal patterns over the membrane. Water droplets reduce the hydraulic diameter of the flow field plate channel, thereby increasing gas flow resistance in the region around the water droplet. Furthermore, because the gas must diffuse around a water droplet, gas diffusion distance is increased. Because gas (reactants and/or oxidants) cannot reach the membrane where the water droplet has formed, that area of the membrane will become inactive and result in distortions of the current density distribution in the fuel cell membrane, thereby reducing fuel cell efficiency. Furthermore, if left standing in a region of the membrane, flow field plate channel and/or other location of the fuel cell, the accumulated water may result in corrosion or deformation to the membrane or other parts of the fuel cell system.